You'll need a plus after print, like this: print+(split(/:/))[0]

(Update: see the beginning of perldoc perlfunc if you don't know why.)

You'll need the perl options and arguments in the shebang line, or make a shell script of this, like

#!/bin/sh
perl -lne 'print+(split(/:/))[0]' /etc/passwd
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or else

#!/usr/bin/perl -ln
BEGIN{ @ARGV = "/etc/passwd"; }
print+(split(/:/))[0]
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or even

#!/usr/bin/perl -w
@ARGV = "/etc/passwd";
$\ = $/;
while(<>) {
print+(split(/:/))[0];
}
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