while (<>) {
   if (/.../) {
      push @matches, ...;
   }
}

##</code><code>##

while (<>) {
   while (/.../g) {
      push @matches, ...;
   }
}

##</code><code>##

my $file; { local $/; $file = <>; }   # Slurp file.
while ($file =~ /.../g) {
   push @matches, ...;
}