You will need wget on your system! See http://www.asitis.org/installing-wget-for-mac-os-x for the Mac. If you have Windows, well, you will need to do something else.

If you want to get all the Perforce files in one step, try this:
#!/bin/bash
cd perforce
mkdir Win32
cd Win32
wget http://www.perforce.com/downloads/perforce/r08.1/bin.ntx86/p4vinst.exe
wget http://www.perforce.com/downloads/perforce/r08.1/bin.ntx86/perforce.exe
wget http://www.perforce.com/downloads/perforce/r08.1/bin.ntx86/p4webinst.exe
wget http://www.perforce.com/downloads/perforce/r08.1/bin.ntx86/p4dtg.exe
wget http://www.perforce.com/downloads/perforce/r08.1/bin.ntx86/p4winst.exe
wget http://www.perforce.com/downloads/perforce/r07.3/bin.ntx86/p4gt.exe
wget http://www.perforce.com/downloads/perforce/r07.3/bin.ntx86/p4ofc.exe
wget http://www.perforce.com/downloads/perforce/r07.3/bin.ntx86/p4rpt.exe
wget http://www.perforce.com/downloads/perforce/r08.1/bin.ntx86/p4ftpd.exe
cd ..
mkdir MacOSX-Intel
cd MacOSX-Intel
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104x86/P4V.dmg
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104x86/p4
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104x86/p4web
wget http://www.perforce.com/downloads/perforce/r08.1/bin.darwin80x86/p4d
wget http://www.perforce.com/downloads/perforce/r08.1/bin.darwin80x86/p4p
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104x86/p4ftpd
cd ..
mkdir MacOSX-PPC
cd MacOSX-PPC
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104ppc/P4V.dmg
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104ppc/p4
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104ppc/p4web
wget http://www.perforce.com/downloads/perforce/r08.1/bin.darwin80ppc/p4d
wget http://www.perforce.com/downloads/perforce/r08.1/bin.darwin80ppc/p4p
wget http://www.perforce.com/downloads/perforce/r08.1/bin.macosx104ppc/p4ftpd